Recently, as personal computers and television receivers have become lighter and thinner, reduction in thickness and weight of display devices has also been wanted. In answer to such demands, flat panel type displays such as liquid crystal displays (LCDs) have been developed in place of cathode ray tubes (CRTs).
An LCD is a display device which produces desired image signals by applying electric fields across a liquid crystal layer having anisotropic dielectric constants, injected between a pair of substrates so that the strength of the electric fields is controlled to thereby control the amount of light passing through the substrates. Such LCDs are typical examples of handy flat panel type displays. Of these, TFT LCDs that employ thin-film transistors (TFT) as switching elements are mainly in use.
Lately, since LCDs have been not only used as the display devices of computers but also used widely as the display devices of television receivers, the need for rendering motion pictures has been increased. However, since the conventional LCDs are low in response speed, they have a drawback that it is difficult to reproduce motion pictures.
In order to make the LCD's response speed problem better, there is a known liquid crystal driving method wherein in accordance with the combination of the input image data of the previous frame and the input image data of the current frame, either a higher (overshot) drive voltage than the predetermined gray scale level voltage that corresponds to the input image data of the current frame or a lower (undershot) drive voltage is supplied to the liquid crystal display panel. In this specification of the present application, this driving scheme should be defined as overshoot (OS) drive.
FIG. 1 shows a schematic configuration of a conventional overshoot drive circuit. Specifically, the input image data (current data) of the N-th frame being about to be displayed and the input image data (previous data) of the (N−1)-th frame being stored in a frame memory 1 are loaded into an emphasis converter 2, wherein the patterns of the gray scale level transitions between both the data and the input image data of the N-th frame are looked up with the applied voltage data table stored in a table memory (ROM) 3 so as to identify applied voltage data, and write-gray scale level data (emphasis-converted data) needed for image display of the N-th frame is determined based on the thus obtained applied voltage data (emphasis conversion parameters) so as to be supplied to a liquid crystal display panel 4. Here, emphasis converter 2 and table memory 3 constitute a write-gray scale level determining means.
The applied voltage data (emphasis conversion parameters) stored in the above table memory 3 is obtained beforehand from the actual measurement of the optical response characteristics of liquid crystal display panel 4. When, for example, the number of display signal levels, i.e., the amount of display data, is 256 gray scales represented by 8 bits, every level of 256 gray scales may have a piece of applied voltage data, as shown in FIG. 2. Alternatively, it is also possible that only the measurements for nine representative gray scale levels, one for every 32 gray scale levels, have been stored and the applied voltage data for other gray scale levels is determined by linear interpolation of the above measurements or other operations.
There has been a problem in that it takes long time to make a transition from a certain half gray scale level to another half gray scale level, so that it is impossible for a general liquid crystal display panel to display the half gray scales within the period of one frame (e.g., 16.7 msec. for a case of progressive scan of 60 Hz). This not only produces afterglow but also hinders correct half gray scale display. Use of the above-described overshoot drive circuit, however, enables display of the aimed half gray scale level within a short time as shown in FIG. 3.
In the case where the liquid crystal response speed is improved by way of the signal processing as above, OS drive is performed by making a comparative operation between the input image data of the previous frame and the current frame data and outputting the emphasis-converted data.
However, if the emphasis-converted data is mis-optimized, errors in data between frames are enhanced, so that video noise which does not originate from due input data will be generated. FIGS. 4 and 5 show the relationships between the applied voltage to the liquid crystal display panel and the transmittance when the input video data changes from black to a certain half gray scale value.
Since in FIG. 4 the emphasis-converted data is optimized in conformity with the liquid crystal display panel characteristic, the target brightness can be realized within one frame, while three frames are needed for the normal drive to reach the target brightness. On the other hand, shown in FIG. 5 is a case where the brightness reaches a level higher than the target because excessive emphasis-converted data is used.
Since the cases explained with reference to FIGS. 4 and 5 are assumed that the input image data changes from black to a certain half gray scale level and continues to be set at that half gray scale level, the output data reaches the target brightness level while the error of the output data is absorbed within one frame. However, if the input data changes repeatedly, e.g., black→half gray scale→black→half gray scale, the error will rapidly increase.
In terms of normally received television signals this problem causes undue images (so-called noise) that are laid over edges such as face contours, character contours, etc., resulting in image degradation such as unnatural hue, white spots, flickering, etc.
Further, when the response speed of the liquid crystal display panel is taken into consideration, it is difficult to output the optimal emphasis-converted data at any time because of variations in cell gap, change in the viscosity of the liquid crystal material due to ambient temperature and other factors.
The present invention has been devised in view of the above problems, and is to provide a liquid crystal display which is capable of eliminating the adverse effects from accelerative drive by detecting edges of the input image and turning on and off the accelerative drive for every pixel.
Further, since in the conventional liquid crystal display shown in FIG. 1, the input image data for the current frame is emphasis-converted and supplied to the liquid crystal display panel, based on the gray scale level transitions of the input image data from one frame to the next, if some noise is laid over the input image data, the noise also is emphasis-converted and supplied to the liquid crystal display panel, causing image degradation such as white spots, flickering etc., resulting from the emphasized noise.
FIG. 6 is an illustrative view showing a case where noise is laid over 3×3 pixels of data. For instance, suppose that noise shown in FIG. 6(b) is added (the pixels of the 135th and 130th gray scale levels are the noise added portions) when data of the 128th gray scale level is supplied to all the pixels as shown in FIG. 6(a). In the normal drive mode, the input gray scale levels are output straight through, so that the display data (write-gray scale levels) shown in FIG. 6(b) is displayed on the liquid crystal display panel.
On the other hand, when OS drive for data emphasis conversion is implemented, this affects the data to enlarge the transition width. So the noise added portions are emphasized to reach the 140th and 135th gray scale levels as shown in FIG. 6(c), hence the noise is displayed prominently. In this way, if a signal source of a poor S/N ratio is supplied to an OS drive configuration, the noise is also emphasized more than that in the normal drive mode, this gives a problem in that the image quality of the displayed image is degraded.
To deal with this, Japanese Patent Application Laid-open No. Hei 3-96993, for example, proposes a configuration, wherein the differential signal as to the video data to be displayed on the liquid crystal display between the current data and the data one frame period or one field period before is detected, and when the magnitude of the differential signal is smaller than the predetermined level, the difference is determined to be noise and the input video data is output straight through, while, when the magnitude of the differential signal is greater than the predetermined level, the input video data is added with the above differential signal so as to output the video data with its afterimage removed.
This scheme is realized by provision of a coefficient circuit composed of a multiplier for multiplying the input signal by a predetermined coefficient or using a ROM table, having an input/output characteristic shown in FIG. 7. More specifically, when the value of the differential signal (motion detection signal) between the pieces of data one frame period or one field period apart, to be supplied to the coefficient circuit, falls within the ranges from 0 to +a and from 0 to −a, or when the magnitude is smaller than the predetermined value |a|, the input video data is output straight through.
On the other hand, when the value of the differential signal (motion detection signal) supplied to the coefficient circuit falls outside of the ranges 0 to +a and 0 to −a, or when the magnitude is greater than the predetermined value |a|, the input signal multiplied by a coefficient having the same polarity as that of the input signal is output and added to the input video data, so that the input video data is emphasis-converted to cancel the afterimage from the image displayed on the LCD device.
However, in the above disclosure of Japanese Patent Application Laid-open No. Hei 3-96993, a coefficient circuit composed of the multiplier or the ROM table is used to obtain output video data in conformity with the magnitude of the differential signal of the video data between the current data and the data one frame period or one field period before, so it is only possible to deal with one-dimensional noise depending on temporal variations. Therefore it has been impossible to prevent image degradation of the displayed image, in a perfect manner.
The present invention has been devised in view of the above problem, and is to provide a liquid crystal display which is capable of positively eliminating the adverse effects from OS drive by enabling switching between the OS drive and normal drive based on multidimensional noise detection result.
In the conventional liquid crystal display shown in FIG. 1, when the emphasizing process (OS drive) by write-gray scale level determining portion 2 is implemented, noise and the like, which are high frequency components, superimposed on the input image data, are further emphasized by the OS drive, posing the image degradation problem in that noise stands out as white spots (in the case of the liquid crystal display panel operated in the normally black mode).
For example, playback of an analog VTR entails noise that is attributed to the tape and head system during signal reproduction, or playback of a tape that is obtained after repeated duplication results in a poor signal to noise ratio producing much noise. If the above-described OS drive is implemented for the input image data superimposed with such noise, even the noise is emphasized and results in image degradation of the displayed image.
Further, when a user who prefers a clear and vivid image adjusts the contour enhancement correcting function of a television system etc., to a severe level, the contour enhanced portions are further emphasized by OS drive to a too strong level and unnatural hues, flickering, etc., arise, degrading the image quality of the displayed image.
Moreover, the video signals for DVD and digital broadcasting are compressed by MPEG-2. In MPEG, it is usually known that the lower the transfer bit rate of codes (the higher the compression rate), the more the coding noise stands out and the more the image quality degrades. As typical coding noise in MPEG, block noise and mosquito noise are well known.
Block noise is a phenomenon whereby boundaries of blocks appear clearly and are seen like tiles. This takes place when the image signal within each block has only low frequency components and the neighboring blocks have different frequency component values. Mosquito noise is flickering noise appearing around edges as if mosquitoes were flying. This noise is generated due to loss of high frequency components that are included in the original image signal, through quantization.
In this way, when coded image data that is encoded based on a coding scheme that implements blockwise orthogonal transformation is input/decoded to perform image display, block distortion whereby boundaries of process blocks appear in the flat portion of the decoded image, and mosquito noise that causes haze around edge portions of characters and contours occur. These noises are emphasized by OS drive, degrading the image quality of the displayed image.
The present invention has been devised in view of the above problem, and is to provide a liquid crystal display which improves the liquid crystal response speed for half gray scale images by implementing overshoot drive while preventing noise etc. from being excessively emphasized, to thereby improve the image quality of the displayed image.
Usually, at the previous stage of the aforementioned overshoot drive circuit, various video adjustments are implemented according to user's preference, hence OS drive (emphasis conversion process) is executed for the input image data which has undergone the video adjustments. Accordingly, depending on the video adjustment result, OS drive may pose a problem in that the image quality of the displayed image is degraded by the occurrence of the adverse effects (unnatural hues, flickering, etc.) therefrom.
For example, when a user who prefers a clear and vivid picture applies rather intensive contour enhancement correction by video adjustment, the contour enhanced portions are further emphasized by OS drive to a too strong level and produce white spots (in the case of a liquid crystal display panel operated in the normally black mode), unnatural hues, flickering and others, resulting in degradation of the image quality of the displayed image.
Since the optical response characteristics of liquid crystal display panels are different depending on the alignment mode of liquid crystal, the electrode structure for applying electric fields across the liquid crystal material and other factors, there exist some gray scale level transition patterns of which the liquid crystal response speed can be well improved by OS drive (emphasis conversion process) and others of which the liquid crystal response speed can not be improved very much by OS drive (emphasis conversion process).
When a picture obtained as a result of the user's video adjustments for input image data as to gray scale level characteristics such as black (white) extension, black (white) level adjustment, brightness adjustment and the like, includes many gray scale level transition patterns of which the liquid crystal response speed cannot be improved very much by OS drive (emphasis conversion process), implementation of OS drive only enlarges data errors between frames, resulting in generation of video noise which does not exist in the original input image data.
Illustratively, there are gray scale level transitions in which the target gray scale level cannot be achieved within one frame even if OS drive is effected. For such transitions, if OS drive is effected for the next frame, the applied voltage of data is determined on the basis that the previous gray scale level has reached the target gray scale level despite the fact the gray scale level has not yet been reached. As a result, gray scale levels which are deviated from due gray scale levels to be displayed are displayed, so that the desired image cannot be displayed. If this is repeated, the error of the output data increase rapidly, posing the problem in that whitened or blackened pixels are reproduced.
The present invention has been devised in view of the above problem, and is to provide a liquid crystal display which can inhibit image degradation due to adverse effects from overshoot drive, by controlling overshoot drive in response to the user's video adjustment for the input image data.
As it has been known that the response speed of liquid crystal greatly depends on the temperature, Japanese Patent Application Laid-open No. Hei 4-318516, for example, discloses a liquid crystal display panel driver that continuously controls and keeps the response speed of gray scale change in an optimal condition without loss of display quality in order to deal with any change of the temperature of liquid crystal display panel.
This configuration includes: RAM for storing one frame of digital image data for display; a temperature sensor for detecting the temperature of the liquid crystal display panel; and a data converting circuit which compares the aforementioned digital image data with the image data that is read out, by a one-frame delay, from the RAM and, if the current image data has changed from the image data one frame before, implements emphasis conversion of the current image data in the direction of the change, in accordance with the detected temperature of the above temperature sensor, whereby display of the liquid crystal display panel is driven based on the image data output from this data converting circuit.
Specifically, suppose that the temperature of the liquid crystal display panel to be detected by the temperature sensor is classified into, for example, three ranges Th, Tm and Tl (Th>Tm>Tl) and three mode signals, corresponding to these ranges, to be output from the A/D converter to the data converting circuit are defined as Mh, Mm and Ml, while in the ROM of the data converting circuit, “3”, the number equal to that of the mode signals, tables of image data, which can be accessed by designating the addresses or the value of the current image data and that of the image data delayed by one frame, are stored beforehand. One table which corresponds to the input mode signal is selected, and the image data stored in the table at the memory location designated by the addresses, i.e., the value of the current image data and that of the image data delayed by one frame is read out to be output to the drive circuit of the liquid crystal display panel.
Next, FIG. 8 is a rear view showing a schematic configurational example of a direct backlight type liquid crystal display. In FIG. 8, 4 designates a liquid crystal display panel, 11 fluorescent lamps for illuminating the liquid crystal display panel 4 from the rear, 12 an inverter transformer for energizing fluorescent lamps 11, 13 a power supply unit, 14 a video processing circuit board, 15 a sound processing circuit board and 16 a temperature sensor.
Of these, items releasing heat that greatly affects the response speed characteristic of liquid crystal display panel 4 are inverter transformer 12 and power supply unit 13. It is preferred that temperature sensor 16 is arranged inside liquid crystal display panel 4, from its due objective, but this is difficult, so the sensor should be attached to another member such as a circuit board.
Therefore, when, for example, the constituents 11 to 15 are arranged as shown in FIG. 8, temperature sensor 16 is attached to sound processing circuit board 15, which is least affected by generation of heat from inverter transformer 12 and power supply unit 13, and the detected output from this temperature sensor 16 is made use of by an overshoot drive circuit provided in video processing circuit board 14.
The above-described conventional liquid crystal display, however, has the following problems.                (1) If, for example, the applied voltage data (emphasis conversion parameters) stored in OS table memory 3 is broken, or the calculation algorithm for linear interpolation or the like in emphasis converter 2 is broken, due to some device trouble, it becomes impossible to supply the liquid crystal display panel 4 with correct applied voltages of data (emphasis-converted data) corresponding to the input image data, whereby the image quality of the displayed image is markedly degraded, thus hindering the attention to the picture.        (2) Further, in the case of the above-described conventional liquid crystal display, in the normal installed state (stand-mounted state) shown in FIG. 9(a) temperature sensor 16 is arranged at the place where it has least influence of heat from inverter transformer 12, power supply unit 13 and other components. However, when the screen is set at the vertically inverted state (in the suspended state from ceiling) as shown in FIG. 9(b) or when rotated by 90 degrees (in the portrait orientation state) as shown in FIG. 9(c), the heat flow path changes hence temperature sensor 16 is significantly affected by generation of heat from the other members, so it is no longer possible to detect the exact temperature of liquid crystal display panel 4.        
As a result, correct applied voltages of data (emphasis-converted data) corresponding to the temperature of liquid crystal display panel 4 cannot be supplied to liquid crystal display panel 4, causing the problem of image quality of the displayed image being significantly degraded by generation of shadow tailing due to application of insufficient applied voltages of data (emphasis-converted data) to liquid crystal display panel 4 or by generation of white spots due to application of excessive applied voltages of data (emphasis-converted data) to liquid crystal display panel 4 (in the case of the normally black mode).
Further, if this liquid crystal display is put in a place where air is blown onto it from a room air-conditioner or in a sunny place or direct sunshine, part of liquid crystal display panel 4 may decrease or increase in temperature, producing varying temperature distribution across the surface of liquid crystal display panel 4. Resultantly, excessive applied voltages of data (emphasis-converted data) may be supplied to liquid crystal display panel 4 in partial areas, producing white spots, or insufficient applied voltages of data (emphasis-converted data) may be supplied to liquid crystal display panel 4 causing shadow tailing (when in the normally black mode), hence image quality of the displayed image is significantly degraded. This problem of varying temperature distribution across the surface of liquid crystal display panel 4 depending on the place of installation becomes more noticeable when the display screen size becomes greater.                (3) Moreover, when coded image data that is encoded based on a coding scheme that implements orthogonal transformation for every block consisting of, for example, M×N pixels, is input/decoded to perform image display, block distortion whereby boundaries of processed blocks appear in the flat portion of the decoded image, and mosquito noise that causes haze around edge portions of characters and contours occur, depending on the compression ratio of the image coded data. When overshoot drive is applied to these noises, the noises are emphasized, resulting in degradation of the image quality of the displayed image.        
Similarly and also, in the case where a picture signal having a poor S/N ratio is input, the noise is emphasized when overshoot drive is effected, causing degradation of the image quality of the displayed image. In this way, depending on the property of the input image, overshoot drive causes adverse effect, thus degrading the image quality of the displayed image.
The present invention has been devised in view of the above problem, and is to provide a liquid crystal display which prevents degradation of the image quality of the displayed image by stopping overshoot drive when an awkward picture is displayed by execution of overshoot drive, due to device trouble, due to the installed state of the device and due to the property of the input image.